CH. 9.2 Life and Geologic Time

Mr. Perez

Important Vocabulary

Adaptation

Geologic time scale

Epoch

Era

Period

Eon

Trilobite

Geologic Time Scale

The appearance or disappearance of types of organisms throughout Earth’s history marks IMPORTANT occurrences in geologic time.

The geologic time scale was made by paleontologists.This time scale divides Earth’s history into time units

based on the life-forms that lived only during certain periods.

The time scale is not only based on life-forms living in certain periods– sometimes, life-forms are not present, so certain time periods of the geologic time scale are based on other information/criteria

Geologic Time Scale

If we were to create a geologic time scale based on the life time of a human, it might look something like this…

Life Eon

Non-Talking Era Talking Era

Crawling Period

Teething Period

School Period Work Period Retirement Period

First tooth Epoch

First molar Epoch

Elementary Epoch

Secondary Epoch

College Epoch

Bus Boy Epoch

Manager Epoch

CEO Epoch

Subdivisions of Geologic Time

The oldest rock on Earth contain NO fossils.

Then, for millions of years after the first appearance of fossils, the fossil record remained sparse.

Later, in Earth’s history came an explosion in the abundance and diversity of organisms.

The FOUR subdivisions of geologic time are:1. Eons

2. Eras

3. Period

4. Epochs

Subdivisions of geologic time

Eons- longest subdivision based on the abundance of certain fossilsExamples- Hadean eon, Archean eon, Proterozoic

eon, Phanerozoic eon,

Era- second longest subdivision based on the major, striking, and worldwide changes in the types of fossils presentExamples- Paleozoic era, Mesozoic era, Cenozoic eraEras are divided into periods

Refer to p. 259 Figure 12

Subdivisions of Geologic Time

Periods- units of geologic time based on the types of life existing worldwide at the time.Examples- Cambrian, Ordovician, Jurassic,

Cretaceous, Triassic, DevonianPeriods are divided into smaller groups called epochs

Epochs- units of geologic time based on differences in life-forms, but some of these differences can vary from continent to continentExamples- Paleocene, Eocene, Oligocene, Miocene,

Holocene

Refer to p. 259 Figure 12

Identify when each development in Earth’s history occurred

Event Eon Era (if identified)

Period (if identified)

First life Archean n/a n/a

First trilobites

Phanerozoic Paleozoic Cambrian

First flowering plants

Phanerozoic Mesozoic Cretaceous

Refer to p. 259 Figure 12

Trilobites

Small, hard shelled organisms

Crawl on the seafloor or swam through water

2cm – 7cm in length, 1cm – 3cm in width

Considered to be index fossils because they lived over vast regions of the world during specific periods of geologic time.

Trilobites Body has three lobes along the length

Head= cephalon

Segmented middle section = thorax

Tail = pygidium

Changing Characteristics of Trilobites

Trilobites lived on Earth for over 200 millions years. Throughout the Paleozoic Era, some trilobite species became extinct and other new species evolved, showing different characteristics.

Trilobite Eyes

Trilobites might have been the first organisms that could view the world with complex eyes…

The position of eyes on an organism gives clues as to where it must have livedEyes on the back of the head indicate a bottom

dwellerEyes on the front of the head indicate an active

swimmer

In most species, trilobite eyes were located midway on the headThis indicates that trilobites could both crawl on the

ocean floor and swim through the water

Trilobite Eyes

As time passed, the eyes in trilobites changed.

In many species, the eyes became smaller Some even lost their eyes completely—

these species probably lived underground and did not need eyes

In other species, the eyes became more complex Aeglina, a species of trilobite, developed

large compound eyes (like insects) Other trilobites developed stalks that held

the eyes upward

Trilobite BodiesTrilobite bodies and

tails also underwent changes in form through time

Olenellus: Lived during the

Early Cambrian Period

Had an extremely segmented body

It is thought to have been a primitive trilobite

AdaptationsAdaptations are characteristics of an organism

that increase its chances of survival in its environment

According to Charles Darwin’s natural selection, a species of animal, overtime, will adapt to its environment…The trilobite’s changing body, eyes, and tail are an

example of an adaptation Charles Darwin studied the beak color and size of

finches in the Galapagos islands

Resources

Florida Science Grade 7

Glencoe Science & McGraw Hill Publishing

Google Images